The present disclosure relates to vent apparatus for use in venting the inside of a rotational mold to outside of the mold.
Rotational molding involves heating a flowable material in a hollow mold and rotating the mold to melt and distribute the material over the inside of the mold. Rotational molding is a high temperature, low pressure process and the strength required from the molds is minimal, which results in its ability to produce large, complex parts using a low-cost mold. Further, the low processing pressure involved in rotational molding has the added advantage of producing parts that are virtually stress free.
Rotational molded articles are used for many different commercial or consumer purposes including but not limited to livestock feeders, drainage systems, food service containers, instrument housings, fuel tanks, vending machines, highway barriers, road markers, boats, kayaks, childcare seats, light globes, tool carts, planter pots, playing balls, playground equipment, headrests, truck/cart liners, and air ducts.
The process of rotational molding generally includes placing a flowable material such as, e.g., a polymer usually in a powder form, inside a mold. Often, the mold is composed of two or more parts and totally encloses the powder. Molds may be made out of steel, aluminum, and/or another metal and may be supported by a steel frame. The mold is then placed in an oven and heated for a predetermined amount of time to allow the flowable material to turn into a liquid state. The mold is rotated in two perpendicular axes throughout the rotational molding process. As the mold heats up, the flowable material begins to coalesce to the inside walls of the mold. The heat distribution around the inner surface of the mold may be determined by the outside design of the mold. For example, tin may be used to reduce heat in areas and gas lines may be used to radiate, or deliver, more heat on, or to, other areas similar to a convection oven.
Centrifugal forces additionally contribute to the accumulation of the flowable material around the inside of the mold (e.g., such centrifugal forces may constantly pull the material against the inside surface of the mold as the mold is rotated about the two respective axes). After a selected period of time, the mold may be cooled. Rotation of the mold may continue throughout the cooling process. Once the flowable material (e.g., polymer) has hardened (after the cooling process has completed), rotation can stop, the mold may be opened, and the mold part can be removed from the mold.
Along with the flowable material, gases (e.g., air, oxygen, nitrogen, carbon dioxide, etc.) are located inside the mold during the molding process. The gases may exercise significant rates of thermal expansion in comparison to the flowable materials inside the mold. Since the mold may be sealed tight, the pressure inside the mold may fluctuate (e.g., increase and/or decrease) due to the temperature fluctuations of the gases located inside the mold during the heating and cooling steps of the rotational molding process. The pressure fluctuations may cause “blowholes” and/or deformations in the article being molded.
To counter these pressure fluctuations, a “vent tube” may be placed in the mold to allow the inside of the mold to “breathe” to the outside of the mold. In other words, the vent tube may allow the pressure inside of the mold to equalize with the pressure outside of the mold. Typically, a wad of furnace filter or steel wool is placed in the vent tube to prevent any flowable material (e.g., polymer) from falling out of the mold through the vent tube as the mold rotates. Tape may also be used to cover the end of the vent tube located inside of the mold. The wad of furnace filter or steel wool and/or the tape may be burned off after a selected time period during a heating cycling of the rotational molding process such that, e.g., the vent tube can breathe. Often, such practices may result in clogged vent tubes, which may cause a resistance in airflow (e.g., which may cause improper molding or blowholes).
Many molds may not fully seal at the points where the mold comes together (which may be called parting lines). Often, the mold may vent through the parting lines prior to the flowable material solidifying, or hardening, and thereby blocking the parting lines. If airflow in a vent tube is restricted after the parting lines have become blocked, the air pressure inside the mold will rise as the temperature inside the mold rises. Likewise, as the mold begins to cool, the pressure inside the mold will begin to fall as the temperature inside falls. During the cooling process, the gas inside the mold may be sealed from the outside of the mold as the polymer completely coats the entire inside of the mold. If the vent remains restricted, a vacuum may be created within the molded part that may cause “blowholes” along the parting lines as gas tries to enter the mold to relieve the vacuum. Additionally, as the polymer hardens during the cooling process, the vacuum may suck a portion, or part of the molded article away from the mold wall and cause a deformed or “scrap” part.
Vents may be used as one-way valves, which may be reliant on the pressure differential of the gas inside the mold to the gas outside the mold, which may provide a positive pressure inside the mold at the end of the heating cycle such that a vacuum may not be created during the cooling phase. Such one-way valve system may have an inability to control pressure that builds up in the mold. A silicone tube used as a vent tube is disclosed in U.S. Pat. App. Pub. No. 2005/0167887 published Aug. 4, 2004 to Rory Jones, which is incorporated herein by reference in its entirety.